Corrosion-inhibitive heat transfer compositions of aqueous alcohol solutions heretofore have widely been employed in heat exchange systems associated with internal combustion engines, solar systems, and the like, to depress the freezing point of the coolant, to raise its boiling point, and provide corrosion protection. Conventional alcohol-based heat transfer fluids, such as automobile antifreezes, have the tendency to corrode the metal (e.g., aluminum, copper, iron, brass and solder) surfaces of the heat transfer system.
There are unique corrosion problems associated with the recent trend to the greater usage of aluminum in internal combustion engine cooling systems. The state-of-the-art antifreeze cooling compositions previously used in such systems have been formulated for use in cooling systems constructed largely of cast iron, employing corrosion inhibitors and parameters calculated to minimize corrosion on ferrous metal surfaces.
Tests designed to simulate the corrosion that would result upon the aluminum structural parts of the cooling system demonstrate that state-of-the-art antifreezes can fail to provide adequate corrosion protection. Indeed, and conceptually, the expected failure can be catastrophic, e.g., such as would result from pinholes in an aluminum radiator.
In automobile cooling systems, any aluminum corrosion that occurs not only damages the aluminum metal surface being attacked by corrosion, but can also adversely affect other parts of the cooling system due to a phenomenon called "transport deposition". By virtue of transport deposition, soluble aluminum corrosion products formed at hot engine surfaces are carried to and precipitated on the cooler radiator tubes, where they form heat-insulating films or coatings that impair heat-transfer.
Two United States patents are particularly noteworthy, as these are considered to be representative of the state-of-the-art antifreeze technology. U.S. Pat. Nos. 3,341,469 and 3,337,496 thus disclose an aqueous alcohol composition employing organosiloxane/silicate copolymers which comprises:
an alcohol; PA1 an organosiloxan/silicate copolymer present in an amount of from 0.01 percent to 10 percent by weight based upon the weight of said concentrate and consisting essentially of: PA1 (1) from 0.1 to 99.9 parts by weight of at least one member selected from the group consisting of (a) siloxane groups represented by the formula: EQU RSiO.sub.1.5 PA1 wherein R is a member selected from the group consisting of the methyl, ethyl, phenyl and vinyl groups and (b) siloxane groups represented by the formula: ##EQU1## wherein Y is a member selected from the group consisting of the cyano group, CH.sub.2 (OH)CH(OH) group, CH.sub.2 (OH)CH(OH)CH.sub.2 group, ##STR1## group, CH.sub.2 (OH)CH(OH)CH.sub.2 O group and R"(OCH.sub.2 CH.sub.2).sub.n (OC.sub.3 H.sub.6).sub.m O group, wherein R" is a member selected from the group consisting of the monovalent hydrocarbons and the hydrogen atom, n and m are integers and n has a value of at least 1, m has a value from 0 to 20 inclusive, the ratio of n to m is at least 2 to 1, a is an integer and has a value of at least 2, C.sub.a H.sub.2a is an alkylene group, the group represented by Y is separated from the silicon atom by at least two successive carbon atoms, b is an integer and has a value of from 1 to 3 inclusive, R' is a monovalent hydrocarbon group, c is an integer and has a value from 0 to 2 inclusive, (b+c) has a value from 1 to 3 inclusive; and PA1 (2) from 0.1 to 99.9 parts by weight of at least one silicate group represented by the formula: ##EQU2## wherein M is a cation that forms a water soluble silicate, d is the valence of the cation represented by M and has a value of at least 1 and e has a value from 1 to 3 inclusive, said parts by weight of said groups in the copolymer being based on 100 parts by weight of the copolymer. PA1 (i) alcohol; PA1 (ii) a corrosion inhibiting amount of an organosiloxane/silicate polymer, that is soluble and stable against gelation in the antifreeze composition concentrate, and that contains, as its essential components, from 0.1 to 99.9 parts by weight of A and from 0.1 to 99.9 parts by weight of B or C, or mixtures of B and C, wherein the individual components of the polymer are defined as follows: PA1 (iii) a buffer selected from the group consisting of ammonium borate, alkanolamine and alkali metal borate, tetraalkyl and tetraaryl-ammonium borates and borate mixtures thereof; ammonium phosphate, alkanolamine phosphates, and tetraalkyl- and tetraaryl-ammonium phosphates, and phosphate mixtures thereof; alkali metal, ammonium, and amine, benzoates, substituted benzoates; salts of the dibasic acids, such as sebacic and azelaic acids, having 6 to 20 carbons, and mixtures thereof; and mixtures of any of the above buffers; said buffer being present in an amount of between about 1 and about 5 wt. percent, based on the weight of the concentrate; to yield a reserve alkalinity from about 10 to 75; and having a pH range in the concentrate of 5.8 to about 7.5, with the proviso that essentially no alkali metal phosphate is present. PA1 (a) diluting the concentrate with water or a mixture of water and alcohol to produce a working antifreeze, and PA1 (b) adding the working antifreeze to an automobile engine cooling system.
Corrosion-inhibitive heat transfer compositions of the type described in the '469 and '496 patents have enjoyed wide usage. A composition of this type typically has superior shelf-life, does not attack rubber parts in cooling systems, is characterized by low levels of foam formation, and is useful over a wide temperature range even after prolonged periods in service. This has provided an antifreeze which, while subject to annoying gelation problems and the like from time to time, has reasonable latitude in preparation and use. Adequate amounts of silicate can be stabilized against gelation by a wide variety of silicones. This is demonstrated by many patents in this field, setting forth a wide variety of silicones considered to be useful.
A commercial antifreeze concentrate within the scope of these patents, phosphate/borate buffered, has a pH of about 10.5 to insure silicate stability. Unfortunately, it has been found that this high pH and the alkali metal phosphate content of the antifreeze can promote transport-deposition of aluminum corrosion products in the radiator. These corrosion products result from cavitation-erosion-corrosion (CEC) at nucleate boiling sites on aluminum, especially when the silicate in the antifreeze becomes depleted.
The high pH of the antifreeze previously referred to is believed to be typical of that utilized by the prior art for antifreezes of this type. The prior art thus urges the requirement of a relatively high pH to prevent the development of silicate gels. U.S. Pat. No. 4,149,985 teaches that the pH at the time of silicate addition to such antifreeze concentrates must be between 9.5 and 10.5 (providing a final concentrate pH of about 11) in order to minimize concentrate stability problems during storage.
For these reasons, a need has developed for an antifreeze capable of providing adequate protection against aluminum and other cooling system metal corrosion and which does not gel in concentrate form. It is believed that this need is becoming more acute, triggered by the desire of many in the industry to have available an antifreeze with such capabilities.
Low pH (below 7.5) silicate-containing antifreeze concentrates have not been used commercially to the knowledge of the present inventor. It is speculated that one reason for this absence may be that the low pH environment would be considered to cause silicate stability problems in the concentrate due to the tendency of silicates to form insoluble gels in this environment, as is expressed in U.S. Pat. No. 4,149,985, previously discussed. A further reason may be the belief that utilizing a relatively low pH may reduce silicate corrosion inhibitor effectiveness since silicates have an inherently lower solubility in this low pH environment. A still further technical reason that low pH antifreezes have not been developed heretofore is the belief that they would be inherently more corrosive toward non-amphoteric metals, such as iron.